Let's go from the very beginning, a very good place to start:
During Yoichi Wada's presidency, the concept of "Fabula Nova Crystalis" was established. FNC was a mythos of its own where three Square Enix talents were selected to direct their own games and make their own interpretations of the lore they were given. These talents were:
* Motomu Toriyama - he worked on the cutscenes for Bahamut Lagoon and FINAL FANTASY VIII, and was tasked on directing FINAL FANTASY XIII, as a FNC entry. He subsequently made the FFXIII sequels, XIII-2 and Lightning Returns. Long story-short, the reason FF13 got sequels was to recuperate the costs incurred in the development of the in-house development engine, Crystal Tools. Crystal Tools is responsible for Tetsuya Nomura's debacle, but we'll get to that in a bit.
* Hajime Tabata - he directed a PLETHORA of portable games: Before Crisis: FINAL FANTASY VII, Crisis Core: FINAL FANTASY VII, The Third Birthday, and finally, his FNC contribution: FINAL FANTASY Agito XIII, then changed into Type-0 to distance the game a bit from the fiasco that turned the FNC mythos with FFXIII's divisive reception.
* Tetsuya Nomura - We know him pretty well, so we'll skip to his contribution to the FNC mythos: FINAL FANTASY Versus XIII, rebranded now into FINAL FANTASY XV, and nothing like the original.
Nomura's original vision for FF Versus XIII was for it to be a trilogy project. Apparently, 90% of the game's script was finished, but Tetsuya was shafted from Versus' development, and many of us speculate that he was given the reins of the FINAL FANTASY VII Remake project as a sort of consolation. The reason for the shafting was because the game never went into full production mode because Tetsuya Nomura's personnel was constantly being called to help work on the Crystal Tools engine, FINAL FANTASY XIII, and FINAL FANTASY XIV. Originally, FXIV wasn't as good as it is now, so Square had to do A LOT OF WORK in order to get it running, seeing as how it was FINAL FANTASY XI's successor, and Square's newest MMORPG.
Nomura's Versus XIII team was the Tokyo Team that had made all of the KINGDOM HEARTS game, up until KHII. With this in mind, he had plans to make the Osaka Team the new KH main dev team, seeing as how Tokyo was busy helping him making his Versus vision a thing. This is why we haven't, nor will we ever see, the Tokyo team developing for KH: they are now throughout the whole company, and some left the company.
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In short: Nomura is salty because his very first ever FINAL FANTASY game never got to see the light of day, and it's the reason as to why KH games post DDD and FF7 Remake project will have elements of aaall of the stuff he wanted to do with Versus.
Personally, I think he wants to do this because, after finishing the FF7 Remake project, he may be too tired to bother working on an original FF game. Though with what we've seen of Verum Rex, and how he bothered to include Yozora in the universe of KH, there's a possibility he may be utilizing KH to test the grounds for a Verum Rex game, and actually develop it into either its own thing, or make it a FF game.
